Abstract: Hydrochemical characteristics play a pivotal role in the site selection and long-term safety assessment of high-level radioactive waste (HLW) disposal repositories. This study utilized integrated hydrogeochemical analysis and modeling to examine the general hydrochemical characteristics, horizontal zoning, and formation mechanisms of hydrochemical characteristics in different hydrogeological zones of the Beishan area in Gansu, China, considering it as a preliminary site for HLW disposal. The results indicated that the predominant hydrochemical types were Cl·SO4-Na and SO4·Cl-Na, with pH values mostly ranging from 7.5 to 8.3. The bedrock groundwater was generally undersaturated with respect to minerals such as halite, gypsum, fluorite, glauconite, and feldspar, while it was often oversaturated with respect to clay minerals. There was a clear horizontal zonation of hydrochemical characteristics from the recharge area to the discharge area. Mazongshan site was the main regional recharge area with low mineralization, where the formation of hydrochemical components was primarily controlled by leaching processes. Sedimentary basins were the discharge areas with high mineralization, where hydrochemical components were mainly influenced by evaporation. The water–rock interaction processes along the flow path were mainly driven by the dissolution of halite and gypsum, with minimal impact on silicates. Overall, the hydrochemical formation of bedrock groundwater in the Beishan area was predominantly governed by evaporation and interaction processes. This research provides important support on hydrochemistry for the site selection of the HLW disposal repository.